1. Field of the Invention
The present invention is related to an over current protection (OCP) method for use in a switched-mode power supply (SMPS) and related device.
2. Description of the Prior Art
A power supply is a power management device which supplies power to electronic devices or elements after performing power conversion. FIG. 1 is a diagram of a prior art flyback power supply 60. A bridge rectifier 62 is configured to rectify an alternative-current (AC) voltage VAC and provide an input voltage VIN to an inverter 64. When a switch 72 is short-circuited, energy may be stored in the primary coil LP of the inverter 64; when the switch 72 is open-circuited, energy stored in the secondary coil LS of the inverter 64 may be discharged to a load capacitor 69 via a rectifier 66, thereby establishing an output voltage VOUT. An error amplifier EA is configured to compare the output voltage VOUT with a target voltage VTarget, thereby generating a compensation signal VCOM. According to the compensation signal VCOM and a current-sense signal VCS, a controller 74 is configured to generate a control signal VGATE for operating the switch 72. The current-sense signal VCS is associated with the inductor current flowing through the primary coil LP.
FIG. 2 is a detailed diagram illustrating the controller 74 in FIG. 1. A clock generator 76 controls a logic unit 79 so as to turn on the switch 72 periodically. Comparators 77 and 78 are configured to limit the maximum value of the current-sense signal VCS to a value which is the smaller one among a peak limit VCS—LIMIT and the compensation signal VCOM. Over current protection may be performed by the comparator 78 and the logic unit 79 so that the current-sense signal VCS does not exceed the peak limit VCS—LIMIT.
However, after the comparator 78 detects that the current-sense signal VCS exceeds the peak limit VCS—LIMIT, there is a signal delay time before the switch 72 is actually turned off. Since the current-sense signal VCS continues to increase during the signal delay time, the current-sense signal VCS often exceeds the peak limit VCS—LIMIT by an amount ΔV which varies with the input voltage VIN. In the prior art power supply 60, the maximum power of the output voltage VOUT may vary with the input voltage VIN, which is an undesirable condition to be overcome.